Document unintuitive MeshBase::clear()
Document unintuitive MeshBase::clear()

We ought to make this a real clear() and make our generation tools use
a new clear_elements() API or some such later, but that will mean a
lot of digging through other code to avoid subtle breaks to
extra_integers support.

jwpeterson/libmesh,BalticPinguin/libmesh,jwpeterson/libmesh,BalticPinguin/libmesh,jwpeterson/libmesh,capitalaslash/libmesh,hrittich/libmesh,hrittich/libmesh,90jrong/libmesh,90jrong/libmesh,hrittich/libmesh,roystgnr/libmesh,libMesh/libmesh,BalticPinguin/libmesh,roystgnr/libmesh,libMesh/libmesh,hrittich/libmesh,BalticPinguin/libmesh,BalticPinguin/libmesh,90jrong/libmesh,libMesh/libmesh,90jrong/libmesh,roystgnr/libmesh,hrittich/libmesh,libMesh/libmesh,capitalaslash/libmesh,90jrong/libmesh,BalticPinguin/libmesh,capitalaslash/libmesh,roystgnr/libmesh,jwpeterson/libmesh,90jrong/libmesh,hrittich/libmesh,capitalaslash/libmesh,libMesh/libmesh,jwpeterson/libmesh,libMesh/libmesh,dschwen/libmesh,jwpeterson/libmesh,libMesh/libmesh,BalticPinguin/libmesh,capitalaslash/libmesh,libMesh/libmesh,90jrong/libmesh,BalticPinguin/libmesh,roystgnr/libmesh,hrittich/libmesh,dschwen/libmesh,capitalaslash/libmesh,dschwen/libmesh,roystgnr/libmesh,dschwen/libmesh,dschwen/libmesh,hrittich/libmesh,capitalaslash/libmesh,capitalaslash/libmesh,roystgnr/libmesh,roystgnr/libmesh,jwpeterson/libmesh,90jrong/libmesh,dschwen/libmesh,hrittich/libmesh,dschwen/libmesh,90jrong/libmesh,dschwen/libmesh,jwpeterson/libmesh